This invention relates generally to the plasma etching of metal conductors, particularly Aluminum, in the fabrication of semiconductor integrated circuits.
Typically such circuits are formed by depositing an Aluminum layer on top of a silicon dioxide layer. A photoresist layer is then applied over the Aluminum layer and photographically exposed to the desired conductor pattern after which portions of the photoresist are removed leaving the desired pattern on the photoresist layer. Thereafter, using a plasma etching technique the exposed portions of the Aluminum layers are removed leaving the desired conductor pattern of Aluminum on the silicon dioxide layer. The remaining resist is then removed and, in the case of multi-level circuits, another silicon dioxide layer is applied, this layer covering the Aluminum conductors and the spaces between the conductors. Thereafter, the process described above is repeated for the second level.
A problem that has been encountered in the manufacture of multi-level circuits is that of ensuring that the silicon dioxide layer deposited on the Aluminum conductor pattern is complete and free of discontinuities or voids or weaknesses, particularly at locations between the conductors. This problem has become more severe as the spacing between the conductors has become smaller.
This problem has been recognized and discussed in the literature, and Messrs. Arikado, Sekine, Okano and Horiike in an IEEE article entitled "Al Tapered Etching Application to Submicron Multilevel Interconnection Process" and published in December 1986 suggest the tapering of the Aluminum side walls to mitigate this problem. The process suggested involves etching using a mixture of CHCl.sub.3 and Cl.sub.2. A polymerized film is continuously deposited on the resist sidewalls during the process and this gives rise to the tapering. One problem associated with this technique is the difficulty in controlling the polymer deposition and hence the final profile.
U.S. Pat. No. 4,412,885 also discloses an etching technique for providing tapered Aluminum side walls in which a principal gas mixture of BCl.sub.3 and Cl.sub.2 is supplemented by a dopant gas mixture of O.sub.2 and a fluorocarbon gas, preferably CF.sub.4. What is not brought out in the above-identified patent is that the angle of taper of the Aluminum side walls is a function of the angle of taper of the side walls of the overlying resist. Accordingly, to control accurately the slope of the Aluminum side walls it is necessary to control the initial slope of the resist walls.
In general, it is difficult to control the resist slope and particularly when the Aluminum conductor lines (and consequently the resist lines thereon) are of the order of 2 .mu.m or less resist slope becomes virtually impossible to control.